This invention relates to plastic molding presses and other types of pressure plates, and more particularly to novel means by which to ensure against application of high closure pressure to the mold sections of a press when a solid impurity is on one of the confronting mold surfaces.
Molds employed in the high pressure molding of synthetic resins, elastomers and other materials are very expensive, and therefore every effort generally is made to ensure against, or at least minimize damage to them.
For example, it is a frequent occurrence that tiny bits of flash or other impurities adhere to one or both of the confronting surfaces of the mold sections, or tiny insert strips or wires become misaligned and thereby overlie a mold face. It has been found that under the high closing pressure utilized with such molds, such obstacles or other impurities literally hob and therefore damage the hardened tool surfaces of the molds.
In view of the foregoing, it has been the practice heretofore to provide for initial closing of the mold sections under very low pressure and to utilize sensitive limit switches to detect whether the mold surfaces are fully closed or are spaced apart due to some intervening obstacle or other impurity, the limit switches under the latter condition functioning to prevent the application of high final closure pressure and thus prevent damage to the mold.
However, it is generally recognized that limit switches as heretofore employed are not sufficiently accurate as a sensing element and they are incapable of being set with sufficient accuracy to detect the presence of impurities of minute size which nevertheless are capable of damaging the mold.